Begining on December 17, as the
final phase of preparation for Christmas, the Church recites or chants
the O Antiphons preceding the Magnificat during Vespers
of the Liturgy of the Hours.

The O Antiphons express the Church's longing and
expectation for the Messiah, her startled wonderment at the fullness of
grace which the Christ-Child is about to bestow on the world. Their theme
is the majesty of the Savior, His wisdom, His faithfulness and sanctity,
His justice and mercy, His covenant with His chosen people, who in their
ingratitude broke faith with Him. They are concerned with His power and
love as King and Redeemer of the world, His relation to every soul as
Emmanuel, God-with-us. (With Christ Through the Year by Bernard
Strasser)

According to Professor Robert Greenberg of the San Francisco
Conservatory of Music, the Benedictine monks arranged these antiphons
with a definite purpose. If one starts with the last title and takes the
first letter of each one — Emmanuel,
Rex, Oriens,
Clavis, Radix,
Adonai, Sapientia
— the Latin words ero cras are formed, meaning, “Tomorrow,
I will come.” Therefore, the Lord Jesus, whose coming we have prepared
for in Advent and whom we have addressed in these seven Messianic titles,
now speaks to us, “Tomorrow, I will come.” So the “O
Antiphons” not only bring intensity to our Advent preparation, but
bring it to a joyful conclusion. (Fr. William Saunders)